trans-Cyclohexane-1,4-diyl bis(4-nitrophenyl) dicarbonate

In the title crystal structure, C20H18N2O10, there are two independent molecules, both of which lie on crystallographic inversion centres. In one molecule the 4-nitrophenyl dicarbonate groups are substituted in equatorial (A eq) positions of the chair-form cyclohexane ring while in the other molecule the substitution is axial (B ax). The dihedral angles between the atoms of the symmetry-unique carbonate group (O=CO2—) and benzene ring for each molecule are 47.3 (1)° for A eq and 11.7 (2)° for B ax. In B ax, this facilitates the formation of a weak intramolecular C—H⋯O hydrogen bond, while the packing is stabilized by weak intermolecular C—H⋯O interactions.

In the title crystal structure, C 20 H 18 N 2 O 10 , there are two independent molecules, both of which lie on crystallographic inversion centres. In one molecule the 4-nitrophenyl dicarbonate groups are substituted in equatorial (A eq ) positions of the chair-form cyclohexane ring while in the other molecule the substitution is axial (B ax ). The dihedral angles between the atoms of the symmetry-unique carbonate group (O CO 2 -) and benzene ring for each molecule are 47.3 (1) for A eq and 11.7 (2) for B ax . In B ax , this facilitates the formation of a weak intramolecular C-HÁ Á ÁO hydrogen bond, while the packing is stabilized by weak intermolecular C-HÁ Á ÁO interactions. = 82.310 (7) V = 974.6 (3) Å 3 Z = 2 Mo K radiation = 0.12 mm À1 T = 150 (1) Table 1 Hydrogen-bond geometry (Å , ).

S1. Comment
The synthesis of title compound is similar to that of cyclohex-2-ene-1,4-diylbis(4-nitrophenyl)dicarbonate (Ali et al., 2008). Here, we used a mixture of cis and trans isomers of cyclohexane-1,4-diol. The trans isomer has been separated from the mixture of cis and trans isomers. Most of the trans isomer remained undissolved in EtOH during the recrystallization at 358 K, after 40 minutes. Pale yellow plates of (I) were obtained after solubilizing this EtOH insoluble solid in dichloromethane. The molecular structure is illustrated in Figs. 1 and 2, showing that one of the two asymmetric molecules possesses equatorial substituents and the other axial. Within the latter, a weak C-H···O interaction (Table 1) occurs. Further C-H···O links help to establish the packing.

S2. Experimental
A solution of 4-nitrophenylchloroformate (5.64 g, 28.0 mmol) in dry dichloromethane (40 ml) was added dropwise via a 100 ml separating funnel into a solution of cyclohexane-1,4-diol (cis and trans isomers) (1.63 g, 14.0 mmol) in anhydrous pyridine (2.15 g, 2.2 ml, 27.1 mmol) and dry dichloromethane (20 ml) in a 250 ml round-bottom flask. A white suspension appeared which was allowed to stir gently at room temperature for 16 h. After this time more dry dichloromethane (40 ml) was added, which dissolved the suspension and then the reaction mixture was stirred for another 6 h.
Then it was quenched by adding deionized water (40 ml). The reaction mixture was transferred to a separating funnel (500 ml), and the lower organic phase was removed. The aqueous phase was washed with dichloromethane (30 ml × 2), and the dichloromethane solutions were combined. These were then washed with deionized water (30 ml × 2), a 1.0% solution of acetic acid (50 ml × 2) and once more with deionized water (40 ml × 2), and then dried over anhydrous magnesium sulfate and filtered. After filtration, the solvent was removed by rotary evaporator. The product was dried in air overnight in a fume hood and then in a vacuum oven for 24 h at room temperature (< 1 Torr). The desired product was obtained in good yield (6.2 g, 84.0%) as a white solid. For recrystallization, the solid was dissolved in 95% EtOH (50 ml) at 358 K, after 40 minutes some of the solid (about 40%) remained undissolved. The warm solution was filtered and the EtOH-insoluble solid was recovered from the filter paper and dissolved in dichloromethane. Pale yellow plates of (I) were obtained by slow evaporation of solvent at room temperature.

S3. Refinement
The H atoms were placed in calculated positions, with C-H = 0.95-1.00 Å, and refined as riding, with U iso (H) = 1.2U eq (C).  View of one of the independent molecules of the title compound with displacement ellipsoids drawn at the 30% probability level. Unlabeled atoms are related by the symmetry operator (-x, -y, 1 -z).

Figure 2
View of the other independent molecule of the title compound with displacement ellipsoids drawn at the 30% probability level. Unlabeled atoms are related by the symmetry operator (2 -x, 1 -y, -z). The dashed line indicates a hydrogen bond. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.30 e Å −3 Δρ min = −0.27 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.